Optional Input Parameters

Input Parameters Omitted from the MATLAB Interface

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Output Parameters

1:
rcond – double scalar

An estimate of the reciprocal of the condition number of AA. rcond is set to zero if exact singularity is detected or the estimate underflows. If rcond is less than machine precision, AA is singular to working precision.

Error Indicators and Warnings

It is possible that info refers to a parameter that is omitted from the MATLAB interface. This usually indicates that an error in one of the other input parameters has caused an incorrect value to be inferred.

Accuracy

The computed estimate rcond is never less than the true value ρρ, and in practice is nearly always less than 10ρ10ρ, although examples can be constructed where rcond is much larger.

Further Comments

A call to nag_lapack_dgecon (f07ag) involves solving a number of systems of linear equations of the form Ax = bAx=b or ATx = bATx=b; the number is usually 44 or 55 and never more than 1111. Each solution involves approximately 2n22⁢n2 floating point operations but takes considerably longer than a call to nag_lapack_dgetrs (f07ae) with one right-hand side, because extra care is taken to avoid overflow when AA is approximately singular.